This invention relates to the analysis, diagnosis and medical treatment of patients, specifically to an improved cannula for isolating, monitoring and measuring exhaled gases that are separated from inhaled gases.
Patients who are ill or undergoing various surgical procedures are often times given oxygen, anesthetic or other treating gases. In many cases, it is desirable and necessary that oxygen or anesthetic be supplied to the patient while simultaneously measuring at least one component (such as carbon dioxide) of the patients exhaled gas. An accurate measurement of the components of the exhalation gases (including end-tidal carbon dioxide) while the patient is undergoing a medical procedure (in intensive care or recovery situations) provides critical feedback for quickly analyzing body functions and prescribing proper treatment.
Previously, attempts were made to measure at least one component of a patient""s exhaled gas from samples diluted by the oxygen treating gas. The apparatus for administering the treating gas did not provide adequate isolation or separation between the treating gas and the patient""s exhalation gases. This caused inaccuracies and resulted in poor correlations between the measured amount of carbon dioxide and the actual amount of carbon dioxide in the patients blood stream.
A number of researchers (including Iberia, et al.; Norman, et al.; Huntington, et al.) Have published technical papers (ex., journal Anesthesiology) addressing this problem. Their approaches, which attempted to measure at least one component of a patients exhalation gas while simultaneously administering a treating gas (oxygen) included extracting gas samples from a patient""s oxygen mask. The results from these approaches did not give accurate correlations between a patient""s level of carbon dioxide measured from exhaled gas and the measured levels of carbon dioxide in the patient""s blood stream.
This problem has been partially solved by Bowe et al. (U.S. Pat. No. 5,335,656, Aug. 9, 1994), but this apparatus still has some significant disadvantages. Bowe et al. described a cannula that uses a wall member located between two hollow nasal prongs to provide a gas tight seal that defines separate inhalation and exhalation manifolds. While this cannula provides separation between the treating gas (typically oxygen) and the patient""s exhalation gases (targeted CO2 sampling components), it also creates an undesirable dead space in which a patients exhaled gases can be entrapped and stagnate. This dead space becomes a potential and likely source for cross contamination between a patient""s current and previously exhaled gases. Whereby, inaccuracies can be introduced into the patients measured levels of exhaled end-tidal carbon dioxide. These inaccuracies can inhibit detection of minute changes in a patient""s body functions, which is critical in intensive care situations. In addition, the wall member located midway between the cannula nasal will reduce the elasticity and flexibility of the cannula body and possibility cause or contribute to patient discomfort.
An alternative cannula is also described by Bowe et al., which eliminates the dead space created from locating a wall member in the cannula body, midway between the nasal prongs. This alternative cannula entails modifying a conventional nasal cannula by cutting or piercing an aperture in the main body of the cannula at the base of one of the nasal prongs. A nozzle-like piece that is substantially more rigid than the cannula material is then inserted through the aperture and into the corresponding nasal prong to form a separate flow channel for gas. This alternative cannula is cumbersome and would require a difficult, time consuming and costly process to manufacture. The alternative cannula configuration would also make it difficult to avoid passing the flexible tubing over the mouth or eyes of a patient when connecting to a source of treating gas or an analyzer. In addition, since the rigid nozzle-like piece is inserted into the cannula""s nasal prong, which will be placed in a patient""s sensitive nasal cavity, it can cause or contribute to irritation and significant patient discomfort.
The improved cannula of the present invention eliminates the problems of the prior-art by providing a continuous, completely isolated flow channel that eliminates the undesirable dead space in which a patients exhaled gases can be entrapped, stagnate and result in cross contamination. Hence, the cannula of the present invention enables more accurate analyses and measurements of a patient""s exhaled gas, while minimizing discomfort.
Accordingly, several objects and advantages of the present invention are:
(a) to provide a cannula that improves the accuracy of the quantitative measurements of a patients exhaled gas;
(b) to provide a cannula that gives a better correlation between the measurements of a patients exhaled gaseous components and the patients actual arterial blood samples;
(c) to provide a cannula that eliminates the undesirable dead space in which a patients exhaled gases can be entrapped and stagnate, resulting in cross contamination;
(d) to provide a cannula with an isolated flow channel that eliminates the potential for a patients exhaled gases to re-breathe, mix or re-circulate, thereby causing contamination;
(e) to provide a cannula that minimizes patient discomfort.
Further objects and advantages are to provide an invention that can be easily and cost effectively manufactured and used with a conventional cannula, manifold, or other components. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.